Powder rolling of nickel-ironcobalt alloys



3,396,015 POWDER ROLLING OF NICKEL-IRON! COBALT ALLOYS Jerry C. La Plante, Hempstead, N.Y., assignor to Alloys Unlimited, Inc., Melville, N.Y. No Drawing. Filed Jan. 11, 1968, Ser. No. 697,037 9 Claims. (Cl. 75-214) ABSTRACT OF THE DISCLOSURE The alloy 29 Ni, 17 Co, balance iron is prepared as a fine powder in the fully annealed condition and is rolled without added binders into a green strip of about 80% density. After sintering at 2000 F. for at least two hours, it is reduced by rolling about 30 to after which it is resintered. By controlling coil and roll diameters, edge breaking, curling and cracking are prevented. A uniform foil as thin as 0.004 in. can be prepared in this way. The lengthy heat treatments are necessary to produce a good product.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates generally to the powder rolling of ,metal sheets and foils and, more particularly, to the production of foils comprised of the alloy 29 Ni, 17 C0, balance iron. This alloy is commonly referred to by the trademark Kovarf other trademarks associated therewith include 'Sealvar, Nilo-K, and Therlo. Alloys with similar, but not identical, compositions include Fernico (28 Ni, 18 Co) and Rodar (0.30 Mn).

These alloys are particularly useful in electrical and electronic applications, because of their low coefficients of expansion over a wide range of temperatures. This suits them for use in glass-to-metal seals, where Corning glass 7055 and 7040 is used, the latter matching the alloys expansion characteristics. The alloy is stamped, drawn or otherwise formed into a large variety of shapes which are used either as an insert between metal and glass or ceramic portions of a package, or as a metallic part of the package itself.

(2) Prior art The use of metal or alloy powders to form relatively massive solid shapes is well known, but use of these materials to form thin sheet products is quite new. Moreover, this technique has been mainly confined to systems where one of the components is a highly refractory material such as tungsten. In a tungsten-based system it could well happen that the temperature necessary to melt all the components is above the boiling point of one of them, so procedures at lower temperatures are necessary. The production of tungsten-based compacts is described in the following US. Patents: Nos. 1,346,192; 2,179,960; and 2,851,381. An apparatus for rolling tungsten and uranium oxide powders into a sheet is described in US. Patent No. 3,245,114.

The preparation of nickel-cobalt-iron alloys into sheet and foil products has heretofore followed conventional practice. The molten alloy is cast into an ingot mold, and the ingot is then successively rolled into slabs, sheet and foil with appropriate intermediate heat treatments.

In any powder rolling operation, it is necessary to produce a green strip that is strong enough to withstand subsequent handling. In the production of very thin sheets and foils, edge cracking, curling, waving and other problems must always be avoided. While these and other problems are present in any powder rolling operation, the solu tion thereof appears to be distinct for each metal or alloy system involved. The present invention is addressed to a nitcd States Patent 0 3,396,015 Patented Aug. 6, 1968 Alloy N ame Fe N1 00 0 Mn Kovar 54 29 17 NllO-k 54 29 17 Ferrrico 54 28 18 Fer nico CG518 53. 8 28.0 17. 9 Fernico C G604 54. 27. 6 17. 9 Radar 53.7 29. 0 17.0 Therlo 54 29 17 (Kovar is the registered trademark of Westinghouse 00., Therlo is the registered trademark of Driver-I-Iarris 00., Rodar is the registered trademark of W. B. Driver 00., and Fernico is the registered trademark of General Electric Co.)

OBJECTS AND BRIEF DESCRIPTION OF THE INVENTION It is a general object of the present invention to provide a method of powder rolling of nickel-cobalt-iron electrical alloys into sheet and foil products.

Another object of the present invention is to provide a method of preparing sheet and foil products of nickelcobalt-iron electrical alloys which is more economical than previous methods.

Still another object of the invention is to provide a method of producing nickel-cobalt-iron electrical alloys in sheet and foil form which avoids ingot casting and rolling.

A still further object of the invention is to provide a method of powder rolling nickel-cobalt-iron electrical alloys that is capable of producing thin foils of the order of 0.004 in. thick.

Yet another object of the invention is to provide a method of powder rolling nickel-cobalt-iron electrical alloys into sheets and foils having at least as good physical properties as conventionally prepared materials of similar compositions.

Various other objects and advantages of the invention will become clear from the following description of the novel method embodied therein, and the novel features will be particularly pointed out in connection with the appended claims.

DESCRIPTION OF EMBODIMENTS The first step in the process is preparation of the alloy powder. The preferred starting materials is a mesh powder with individual particles being spherical in shape. This has been found adequate for producing a 0.004 in. foil, but a thinner foil would require a smaller particle size powder, inasmuch as a 0.004 in. foil is close to having a single particle thickness at 100 mesh.

The powder must be fully annealed to the soft condition. This can be accomplished by heating at 1300- 1350 F. for about 1 to 3 hours. Those skilled in the art will appreciate that all heating steps during the process must be carried out in a neutral or reducing atmosphere to prevent oxidation. Forming gas is satisfactory. The anneal in a reducing atmosphere removes any surface oxides in addition to rendering the material in the soft or fully annealed condition. Unless the anneal is carried out with the particles suspended in a gas stream or the like, there is bound to be some agglomeration of the particles, and it is necessary that the powder again be brought to --100 mesh. Any suitable milling apparatus that will not contaminate the powder can be used for this purpose. The powder is ready for rolling when it has a Hall flow rate of 15 to 30 seconds. The Hall flow meter determines both apparent density and flow rate; a 50 gram sample of the powder is passed through a funnel having an orifice in. in diameter and Vs in. deep. The flow rate is the time the sample takes to pass therethrough (see ASTM standards B212 and B213).

In carrying out powder rolling in accordance with the invention, no binders are used; the powder is poured into the rolls without any additives or moisture whatsoever. Apparatus for powder rolling is known in the art and need not be described in detail herein. A rolling mill having the axes of the rolls in a horizontal plane is provided, and width of the green strip is controlled by metal guides which fit down over the rolls and somewhat into the nip. The powder feeds from a hopper with sufficient static head to assure continuous flow.

The most important parameter to control during rolling is the density of the green strip, which should be about 80%. If density is too low, of course, the strip will not have structural integrity and will fall apart. It has been found that strip produced at high densities is likely to be uneven in density, which affects the quality of the finished product. While it is difficult to quantify the allowable variations in green strip density with exactness, a variation of over the desired 80% figure is considered tolerable. This is controlled by adjusting roll gap and roll speed.

The minimum and maximum green strip thickness that can be obtained while still achieving the required density are not known. Satisfactory green strips having thicknesses of from 0.03 in. to about 0.06 in. have been produced. It is doubted that a good product could be produced above about 0.1 in. Or below 0.01 in.

The green strip must be sintered for at least 2 hours at a temperature of 2000 F. or higher. While the speed with which the strip can be produced is not very high, the length of the sintering operation makes it impractical to sinter on a continuous basis. Thus, the green strip must either be cut into pieces which will fit the sintering furnace, or it must be coiled. It has been determined that green strip of the proper density can in fact, be coiled, provided that the coil radius is at least 200 times the thickness of the green strip. For example, a 0.04 in. green strip can be wound on a 16 in. diameter coil without breakage. It is preferred that there be some loose alloy powder on the surface of the strip during sintering, as this helps prevent sticking. The coiled strip should be placed on a perforated plate during sintering so that any of this loose powder which falls off during the operation does not itself stick or cause the coil to stick. It has been found that sintering the green strip for at least 2 hours at 2000 F. or higher is essential for production of a finished product having a sound metallurgical structure.

In most powder rolling operations the sintered strip is hot rolled to produce densification. With the present invention this has not been found necessary. A single pass reduction of at least 30% and preferably 30 to 50% is carried out on conventional equipment. To produce a superior product, it is preferred that reduction rolling be carried out in a mill having rolls that have a diameter about 100 times the green strip thickness (i.e., 4 inches for a 0.04 in. green strip). This gives good protection against curling and edge cracking. Larger rolls can cause snaking camber due to lateral restriction in the roll throat. Smaller rolls can cause excessive waving due to density differences in the strip.

After rolling, the strip is again sintered for at least 2 hours at a temperature of 2000 F. or higher. The same precautions against sticking should be observed as in the first sintering step. It will be appreciated that the strip at this stage is much stronger than the green strip, and no particular handling precautions need be taken. This second sinter results in diffusion bonding between regions of previous voids. At this point, the strip is substantially 100% of theoretical density.

From this point on, processing of the strip to produce the final product is done by conventional methods. For example, if the sintered strip is 0.02 in. thick, it may be rolled in two stages with an intermediate anneal down to a 0.0085 in. final thickness. Such a finished strip has been successfully drawn into TO-18 headers which were sealed to appropriate bases. The seals were good and met all specifications.

As noted hereinabove, foil and sheet products made in accordance with the method of the invention exhibit physical properties at least as good and in some respects better than similar products produced by conventional techniques. This is illustrated in the following tabulation, wherein published data for Kovar is compared with data on a piece of powder-rolled material.

(p.s.i.) 75, 000-80, 000 Elongation, percent 35 Density, lb./in.

It will be understood that various changes in the details, steps, materials, compositions and arrangements of parts may be made by those skilled in the art within the principle and scope of the invention as defined in the appended claims and their equivalents.

What is claimed is:

1. A method for producing thin sheets of nickel-cobaltiron electrical alloys comprising:

(a) providing said alloy in powder form with a 100 mesh particle size and in the fully annealed condition; (b) compacting said powder in a rolling mill to form a green strip having about of theoretical density;

(0) sintering said green strip under non-oxidizing conditions for at least two hours at a temperature of at least about 2000 F.;

(d) reducing the thickness of the sintered strip by about 30% to 50%; and

(e) sintering the reduced strip again under non-oxidizing conditions for at least two hours at a temperature of at least about 2000 F.

2. The method as claimed in claim 1, wherein the powder provided in step (a) has a Hall flow rate of about 15 to 30 seconds.

3. The method as claimed in claim 1, wherein the powder provided in step (a) is annealed at from 1300 to 1350 F. for about 1 to 3 hours in a non-oxidizing atmosphere.

4. The method as claimed in claim 1, wherein the density of said green strip is between 75% and of theoretical.

5. The method as claimed in claim 1, wherein the thickness of said green strip is between 0.01 and 0.1 inch.

6. The method as claimed in claim 5, wherein the green strip is wound in a coil having a radius of at least 200 times the green strip thickness prior to step (c).

7. The method as claimed in claim '5, wherein step (d) is carried out in a rolling mill, and the rolls of said mill have a diameter of about times the green strip thickness.

8. The method as claimed in claim 1, wherein said alloy has a nominal composition, by weight, of 29% nickel, 17% cobalt and the balance iron.

9. A method for producing foils of an alloy having a nominal composition, by weight, of 29% nickel, 17%

cob-alt and the balance iron, from a powder of said alloy, comprising:

(a) annealing said powder at about 1300" to 1350" F. for lfirom about 1 to 3 hours in a reducing atmosphere;

(b) size reducing the annealed powder until it has a Hall flow rate of about to seconds;

(0) compacting said powder in a powder rolling mill, in the absence of any added binders, to form a green strip having a density, as compared to theoretical density, of :5%, said green strip having a thickness of between 0.01 inch and 0.1 inch;

(d) winding said green strip in a coil having a radius of at least 200 times the green strip thickness;

(e) sintering the coiled strip in a non-oxidizing atmosphere for at least 2 hours at a temperature of at least 2000 F.;

(f) rolling the sintered strip in a rolling mill to reduce its thickness by about 30 to 50%, thereby densifying 6 it, the roll diameter being about times the green strip thickness;

(g) sintering the reduced strip in a non-oxidizing atmosphere for at least 2 hours at a temperature of at least 2000 F.; and

(h) tfor-ming the strip produced in step (g) into a foil of desired thickness.

References Cited UNITED STATES PATENTS 2,827,407 3/1958 Carlson 75-224 X 2,842,471 7/ 8- Koehlel' 75-221 X FOREIGN PATENTS 202,202 6/ 1956 Australia.

CARL D. QUARFORTH, Primary Examiner.

A. J. STEINER, Assistant Examiner. 

